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Best HVAC for a 1,100 sq. ft. passive solar house at 46 N latitude?

Randy Bunney | Posted in Mechanicals on

We are building high-performance, all electric, 1,100 sq foot passive solar home in cold weather winter climate with hot, humid summers. Slab on grade. Exposed concrete floor as thermal mass. Natural gas is not available. Winter temps infrequently reach -20 to -30 degrees range in December and January. We will use modeling to size heating and cooling system. We will have two bedrooms, open living, dining, and kitchen (primary solar gain area). In addition, there will be a full bath, 1/2 bath, mudroom, and mechanical room. Heating days the last 3 years: 7,065; 8,980; 9,799.

Current HVAC Plan features Mitsubishi variable refrigerant flow (VRF) air source heat pump. Heat pump is effective to -13F. Electric furnace will be used for backup for temps below the heat pump range. Our design team concludes that cabin’s small footprint does not warrant a heat pump with multi-indoor units for zone heating

Instead the design calls for running a single heat pump coil into plenum for electric resistance coil. This plan offers initial cost savings over multi-splits. Duct work will transport heat to individual rooms. We will use a ultra small marine wood stove for backup in case of power failure

Alternate HVAC Plan Utilize efficiency and comfort of being able to control conditions in specific rooms offered by multi-split heat pump. Pros would determine how many indoor units. Electric furnace as backup for temps below the heat pump range

We have a good design team open to suggestions. Subsequently, I welcome your comments or suggestions regarding Current HVAC Plan compared to Alternative HVAC Plan or other alternatives. Thanks.

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  1. Jin Kazama | | #1

    Hi Randy ,
    46lat? care to divulge approximate location ?

    Mini-splits are hard to touch right now in efficiency territory.
    Why a plenum ? you plan on installing an air duct system somwhere in there ? Please note that in such a small Passivhaus building, heat loads will be very low. You should keep it as simple as possible. Look at in-floor electrical heating instead of baseboards if you do not want anything to show, you will not need high power version and should be pretty cheap to include in your concrete floors.

    Depending on your electricity rates/generation, wood should be your friend during those lower than -20c nights where the COP of the minis plummet and the loads are high.

    If you need more than 1 mini heads, don't use a multi-head setup; buy 2 separate small units. It usually ends up the same price; you always have a backup if 1 fails, and the efficiency is higher on most individual models.

  2. Randy Bunney | | #2

    Jim, North Central Minnesota lakeside with strong lake breezes during summer. Trees on north to help protect from winter winds. Trees on east to protect from undue solar gain in summer. Wide open to the west. Site orientation is about 210 degrees.

    Yes, design team envisions air duct system to distribute heat pump heating & cooling, as well as output from electric furnace during backup. I failed to mention in floor electrical heating in main bath. Perhaps 1/2 bath too. However, your comments certainly will nudge me to look into additional in-floor electrical.

    We are far short of passivehaus standards. Current wall assembly plan is 2x6 studs 24 inch centers with blown cellulose and rigid insulation on exterior. I'd like to reach R40+ in walls. My preference would be rock wool, instead of rigid foam, for environmental, fire and bug resistance, and stable R value, as I understand it, at temp extremes.. We haven't detailed roof assembly yet, as we are in preliminary stages of design and are just now establishing baseline for cost estimates.

    Aspiration is that our budget will allow us to reach at least R60 in roof, with triple pane Marvin windows on at least south and west walls. Unheated garage will cover most of east wall. Few windows in north. Structure is double shed roof with clerestory windows over vaulted living/kitchen/dining area for daylighting, additional solar gain in winter, and for night flushing heat (stack effect) from floor's thermal mass during summer.

    Constructing near air tight is standard practice for our builder.

    Thanks for the tip on not using multi-head setup. I would think that we would experience potentially uncomfortable variations in room temps with only two indoor units. Builder and architect warn about wood stove overheating building. So I have located a tiny marine wood stove 11" x 13" x 18" with 4 inch stove pipe.

    Thanks again. I learn a ton from this site.

  3. GBA Editor
    Martin Holladay | | #3

    If your house has an excellent thermal envelope -- above-code levels of insulation, as few air leaks as possible, and high-performance windows -- you can certainly heat your house with a single ductless minisplit heat pump, as long as:

    1. Your family members are willing to leave their bedroom doors open during the day, and

    2. Your family members are willing to tolerate room-to-room temperature differences of about 5 degrees on some nights.

    If you go this route, you should include a little bit of electric resistance backup night to handle severe cold spells.

    If you are worried about room-to-room temperature differences, you should consider installing a ducted minisplit unit.

    Here are links to the best two articles to help you design your heating system:

    Minisplit Heat Pumps and Zero-Net-Energy Homes

    Practical Design Advice for Zero-Net-Energy Homes

  4. Randy Bunney | | #4

    Thank you, Mark. I read both your articles. I now understand better the implications of ducted and non-ducted heat pump. There is so much to learn. Detailed HVAC tech discussions are often beyond me. But I can pick up key principles, such as the doors-open or doors-closed impact.

    One remaining question: Am I correct in assuming that even with the air exchange that occurs with a heat pump, that I will still need HRV or ERV?

    Meanwhile, wouldn't it be something if we had measurables over time detailing the impact nationwide that the apparent surge, albeit likely modest at this point, in high performance building is having upon energy usage.

  5. GBA Editor
    Martin Holladay | | #5

    Q. "Am I correct in assuming that even with the air exchange that occurs with a heat pump, that I will still need an HRV or ERV?"

    A. A tight home always needs a ventilation system -- but not necessarily an HRV or an ERV. You may want to consider either an exhaust-only ventilation system or a supply-only ventilation system. For more information, see Designing a Good Ventilation System.

    Air-source heat pumps (including ductless minisplits and ducted minisplits) do not move any outdoor air to the interior of a house, nor do they move any indoor air to the outdoors. The air that blows through the indoor fan-coil unit is just indoor air that is recirculating past the copper coils.

  6. Randy Bunney | | #6

    Thank you, sir. And happy New Year to all.

  7. Steve Vigoren | | #7

    I plan to build a similar home one hundred or so miles to the north of you in two years. I plan to have an HRV which exhausts from the bedrooms/baths and supplies to the living areas so there will be a circulation of air to/from the areas. Cut an inch off the door bottoms for those times the doors are closed. I am planning for ductless mini splits and Electric heat in the floor (eliminates the furnace and duct work and space they take up). Good luck

  8. Randy Bunney | | #8

    Steve, love to connect and compare notes. We are shooting to break ground next Aug/Sept. Six one two. Two six nine and Forty seven Forty two. Anytime. Thanks. Randy

  9. Expert Member
    Malcolm Taylor | | #9

    Randy, Remember to check that the marine stove and chimney meets your local building codes.
    Good luck with your build!

  10. Jin Kazama | | #10

    what will be the length of the building ?
    With 2 mini-splits heads located at each end of the building, i don't see how you could end up with more than 2-3F of difference all over the place. You then setup individual thermostats in closed rooms with a desired minimum set point ( let's say 1-3F lower than the minis setup ) and be done, the backup will only start up on the cold nights and you will never experience more than ~2F of difference all around.

    From my personal experience of heating 2 floors each of 1600sqft with mini-splits located at each ends, setting the temp 1c higher than usual makes a great impact on comfort and the energy usage ends up nearly identical as the resistance backup heating kicks in further in the night.

    Remember the more insulation you add, the more stable and distributed your temperatures will be.

    Rockwooll is a great choice, but as far as i know ( never got any answer back from Roxul rep ), we still have no data on how rockwool insulation varies according to temperature under 0c ( correct me if i'm wrong someone .. here is some reading about this : and the subsequent discussions ) so please plan your R value during coldest temperatures carefully.

  11. Randy Bunney | | #11

    Jim, Building envelope is 28' x 40' with latter length being the southerly exposure. The great room is our main solar gain area (open living, dining, kitchen) and is 14' ft wide x 28' long and is positioned on the north south axis. A bedroom is on either side of the great room on the south. Northern end features main bath to the west and mudroom with airlock entry to the east.

    My wife and I are elderly and indoor comfort is way up there in priorities. ~2F variance in temps is for sure in the comfort zone. I have to remember that the backup would only come on rarely.

    Now, I think I get it, Jim! Individual thermostats in closed rooms are for the backup in-floor electric heat.That makes sense and would give us the zone heating that I have been shooting for to ensure comfort level during times of extreme outdoor winter temps. Thank you.

    Malcolm, the marine stove I have located is startling small: One has to wait about 1 year after order for delivery. 10,000 BTU's Low to 28,000 BTU's High. A similar size stove, the Hobbit, is an English import. . Downside of both is the maximum length of 12" logs (at least for the Little Cod).

  12. Randy Bunney | | #12

    Just to round out the discussion from this thread ...

    Electric heat in slab as backup to heat pump will give us a more comfortable radiant heat than forced air. The normal downside of cost to operate electric radiant heat will be a minimal factor, as we will only use in cases of extreme cold as backup to heat pump.

    Moreover, as a homeowner, I will have to be aware that the electric radiant heat will require time to come to desired temp. This brings up controls. My understanding is that options includes dual sensors (slab and ambient), air only thermostat, or slab only. I wonder too if learning thermostats, such as Nest, will offer advantage. And finally, I am particularly interested in controls that offer wi-fi capability for remote control of multiple zones in case we want to leave the building unattended during the winter.

    Meanwhile, it is so helpful as a homeowner to tap into the expertise at greenbuildingadvisor

  13. Expert Member
    Malcolm Taylor | | #13

    Randy, that' a neat little stove! My concern came from our codes which specify a minimum diameter for the chimney flue.
    Have you thought about using radiant cove heaters instead of heated floors for your backup heat source? They still have same nice attributes of radiant heat but with an instant response time.

  14. Randy Bunney | | #14

    Oh, my! Just when I think I have a picture in my head of the best HVAC design for our cabin, along comes your notion of cove heaters, Malcom! Actually, thanks. Very intriguing idea for backup heat alternative to electric radiant slab. I could put the installation cost savings from cove heaters into more insulation. :-)

    Especially, given the following ...

    Martin Holladay's article "All About Radiant Floors." quoting Alex Wilson

    If a concrete slab is ‘charged’ with heat [via hydronic tubing] during the early morning hours and the surface is warmed to the point where it cannot readily absorb solar radiation striking it, that solar heat will more directly heat the air, increasing the risk of overheating,” Wilson wrote. “This isn’t a huge problem with radiant-floor heating systems, but it may mean that homeowners will have to open windows periodically in the winter and their overall energy savings from solar energy will not be as great. [Andy] Shapiro counsels against the use of radiant slabs in areas of houses with passive solar heat. ‘It’s a waste of energy,’ he says, though just how much waste occurs is unclear.

    Stove Pipe Meanwhile, Malcom, our rural cabin will never see a building inspector and there are no codes to meet other than state electrical inspection, maybe plumbing, and approval of our site plan for setbacks, etc. I wonder if the code you mention for minimum chimney flue pertains to larger wood burning stoves. These tiny marine stoves come standard with 4" double walled pipe and are compatible for small, wooden vessels at sea -- not a place where one wants to deal with hazards from malfunctioning wood burning stove.

    Best from the Midwest,

  15. Ven Sonata | | #15

    Randy, sounds like a nice set up. Glad you are thinking about woodstove back up. If you lose your power then no problem. All electric houses are desperately vulnerable if the grid goes down like the Quebec ice storm. People were hooped for weeks. So I will suggest that tiny wood stoves are for boats and tents, not for 1100 sq ft houses. Install a moderate size high efficiency metal stove. The few days you will need it will be 30 below zero. Luxuriate, crack a window if it gets too hot the btu loss is so trivial with wood heat you can't measure it. You won't use half a cord a year. But if you do have a serious outage you are laughing. By the way I agree with the others. I live in a super insulated house exclusively headed with wood stove and no ducting. The heat evens out within a few degrees. Put transom over your bedroom doors if you are worried then you have privacy and air flow. Transoms were the 19th century electricity required. They work just fine with single source heat pumps.

  16. Randy Bunney | | #16

    Ven, no toy stoves, eh? Point well taken. Thank you.

    Our design team will be doing load calculations soon, so we'll be able to zero in on projected loads instead of my guessing. Oh, yeah, I like the transom idea. And I am glad that your system is working well for you, Ven.

    Our strategy with electricity, mixed with a measure of hope, is that the cabin will be ready in the decades ahead to take advantage of more renewable energy coming online. Who knows? With increased availability HVAC controls via smartphones we may even get a chance to slip away during the worst of Minnesota's winter, leaving our home unattended for extended times .

    And here's to a future when building green will not be so complicated for consumers and HVAC will be more standardized.

    Meanwhile, best to everyone for the new year. And thank you all for sharing your knowledge.

  17. Jeff Carroll | | #17

    If you go steel stove, you might consider the Englander 17-VL. Yes, it's sold by Home Depot, but it's a well-designed steel stove with a secondary burn. Check out reviews on forums outside HD. It also has the kit for external air hook-up.

    Granted it has a contemporary look, which fits well with the house we're building this year.

    For us it's a 1780 sq. ft. 1.5 story with loft house in the Missouri woods near Hannibal. We're looking at two mini-split's, also, with a 15K BTU until on the lower level and a 9K BTU in the loft, at opposite ends of the house. Still decided whether to go ductless or ducted in the loft. Electrical resistance in the master bath, and a few sitting around for backup, along with the wood stove. We'll be exhaust-only ventilation.

    If you want to talk about builds, feel free to drop me a line at [email protected]. Love to talk to a fellow owner-builder. Best of luck!

  18. Beth Turner | | #18

    We are building a house that is *remarkably* similar (1100 sf, 2 bedrooms, open floor plan, same insulation levels, just north of you near Winnipeg). After extensive research, we've decided to go with the Pacific Alderlea T4 woodstove. It's a little oversized for our heating loads, but we will be burning mostly softwood, and the mass of the cast iron should help to even out the heat a little. In terms if a backup/supplemental system, we have run the gamut of options. The plans we have drawn up are for a slab on grade foundation, but for a number if reasons, I think we are going to change it to a crawl space (icf wall). Originally the plan was to use cove heaters, and possibly add a minisplit down the line. Recently, we have been exploring another idea...While a regular in-slab pex system would likely be overkill for our house (and too slow to respond), we are wondering about a more responsive, low temperature radiant hydronic system, like the European style radiators, or an over-floor system (like Warmboard, but one of the cheaper DIY versions). The advantage of this is that we'd have a distribution system in place that could be heated by any number of fuels/systems in the future--we would not be stuck with electricity as we would be with electric resistance. That said, for now, we would prefer to go all-electric, and are wondering if anyone has any experience with/opinion about this electric combi-boiler: We wondered if, by combining the DHW heater and heating system, we might achieve some efficiency gains. Any thoughts? Is this a bad idea?

  19. Aj Builder, Upstate NY Zone 6a | | #19

    Jeff, Randy, I like that small HD stove, perfect for this home and many others.

  20. Randy Bunney | | #20

    Hi, Beth, thanks for posting.

    I have great respect for efficient home builders in your latitude. The electri combi-boiler looks very interesting. I suggest posting your question as a new Question on this forum. I think your post will be seen by more viewers, as my original question has about run its course in terms of responses. Good luck up north.

  21. Ben Newell | | #21

    We've been working with Efficiency Vermont for a few years now on their high performance manufactured homes, which are in this size range. Here is a GBA article on the homes
    Contact me if you'd like more information: ben at

  22. Eric Habegger | | #22

    @comments #4,5,and 7. No you don't need an HRV or ERV. But like was said then you need a supply or exhaust ventilation system. An inch under the door works to some extent. A better way is to use the system approach. The house I'm remodeling, which also happens to be 1100 sf, will be using an exhaust ventilation system using a Panasonic fan in the one centrally located bathroom.

    I decided that the best option using that method is to be able to seal off the bathroom door - no cutoff at the bottom of that door. Since my bathroom has one bedroom on each of the two parallel bathroom walls, I'm putting in a vent near the floor in each bedroom that tunnels through the wall into the bathroom. Since the bathroom will be designed to have very low air leakage 95% or more of the air movement from the bath fan in the bathroom will come from air in each bedroom. You can then use the undercut doors in the bedrooms to pull warm air from the main family/living room where the heat pump resides. There shouldn't be any need for more than a centralized heat source with this setup.

    Of course, this wouldn't work if your bedrooms are on opposite sides of the house. Also you have to engineer the vents in the bathroom so you maintain privacy. I plan on using in wall ducts so it will end up being shaped a little like a periscope with no sight line.

    I guess I should add that you pretty much will always keep the bathroom door closed for this system to be an effective whole house ventilation system. You'll save a ton of money over an HRV/ERV in both the component and electrical use. One would have to assess one's climate individually but in many cases in small single floor homes I think it would work to replace standard forced air systems with ducts using a centralized heat source or air conditioner. This is an ongoing experiment of mine.

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